Extensive work has been reported in the patent literature on the use of hot, high pressure hydrogen for vaporization of used motor oil. In particular, the assignee herein, Marathon Ashland Petroleum LLC, owns Moore U.S. Pat. No. 6,402,938 which describes re-refining used motor oil by direct injection of a superheated, non-hydrogenating recycle vapor, and the Schaffer Jr. et al. U.S. Pat. No. 6,402,937 which is directed to a process for the direct contact heating and vaporization of used motor oil.
While superheated steam has been used to recycle used motor oil, until the present invention there has been no disclosure of using superheated steam distillation to increase the softening point and carbon yield of pitch.
Heavier hydrocarbons, such as petroleum pitch are used as a carbon precursor for many applications. One important characteristic of carbon precursors is the “carbon yield.” Most commercially available petroleum pitches currently exhibit a carbon yield (as measured by the Modified Conradson Carbon method, ASTM D 2418) of 50 wt % or less. Pitch is used as a carbon precursor for the production of graphite electrodes, carbon fibers, carbon/carbon composites and the like. The efficiency of many of these applications is increased as a function of carbon yield.
Ward et al. (U.S. Pat. No. 4,927,620, Ward, et al., “Process for the manufacture of carbon fibers and feedstock therefore,” May 22, 1990) teaches that the carbon yield of pitch can be increased by removal of the more volatile components of pitch via distillation techniques. To prevent undesirable changes in the pitch product due to thermal treatment, a short path distillation technique using a wiped film evaporator is utilized. Pitch product obtained from this process can have a softening point up to approximately 275° C. A corresponding increase in carbon yield as measured by the Modified Conradson Carbon method also increases up to approximately 80, and in some embodiments, 82 to 85 wt %. Successful implementation of this technique requires the use of extremely good vacuum during processing.
The injection of normal steam into distillation columns is known to reduce the effective vapor pressure of hydrocarbons (J. L. Kroschwitz, M. Howe-Grant, editors, “Steam Distillation,” Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, John Wiley & Sons, New York N.Y. 1993, Volume 8, pages 348, 349). The use of superheated steam has been demonstrated as a heat transfer medium in the production of coal tar and petroleum pitch (Tsuchitani, et al., U.S. Pat. No. 4,925,547, “Process for producing pitch for the manufacture of high-performance carbon fibers together with pitch for the manufacture of general-purpose carbon fibers,” May 15, 1990).
It would be advantageous to provide an efficient method for increasing the softening point and also increasing the carbon yield of heavier hydrocarbons in a continuous manner.
It would also be advantageous to provide an efficient method for increasing the softening point and increasing the carbon yield of heavier hydrocarbons which does not require apparatus susceptible to clogging or fouling under normal processing conditions.
Until the present invention, however, there has not been a process that can be used to increase the softening point of heavier hydrocarbons by direct injection of superheated steam into the heavier hydrocarbons.
Until the present invention there has also not been a process that could be used to increase the carbon yield of heavier hydrocarbons by direct injection of superheated steam.
We devised a superheated steam distillation process which provides an effective short residence time distillation of heavy hydrocarbons without causing an excess of thermal treatment.